Add the possibility to rename abbreviations

The abbr function doesn't have the possiblity to rename abbreviations.
You have to delete the old one and create a new one. This commit adds
this functionality and uses the syntax:

abbr -r OLD_KEY NEW_KEY

Fixes #2155.

tests/abbr.err

@@ -1,2 +1,7 @@
 abbr: no such abbreviation 'NOT_AN_ABBR'
 abbr: abbreviation cannot have spaces in the key
+abbr: no such abbreviation '__abbr6'
+abbr: abbreviation cannot have spaces in the key
+abbr: option requires an argument -- -r
+abbr: Unexpected argument -- __abbr10
+abbr: abbreviation '__abbr12' already exists, cannot rename

tests/abbr.in

@@ -37,3 +37,35 @@ abbr d2 env a=b banana
 abbr -l | string match -q d2; or echo "= test failed"
 
 abbr "a b c" "d e f"; or true
+
+# Test renaming
+abbr __abbr4 omega
+abbr | grep __abbr5
+abbr -r __abbr4 __abbr5
+abbr | grep __abbr5
+abbr -e __abbr5
+abbr | grep __abbr4
+
+# Test renaming a nonexistent abbreviation
+abbr -r __abbr6 __abbr; or true
+
+# Test renaming to a abbreviation with spaces
+abbr __abbr4 omega
+abbr -r __abbr4 "g h i"; or true
+abbr -e __abbr4
+
+# Test renaming without arguments
+abbr __abbr7 omega
+abbr -r __abbr7; or true
+
+# Test renaming with too many arguments
+abbr __abbr8 omega
+abbr -r __abbr8 __abbr9 __abbr10; or true
+abbr | grep __abbr8
+abbr | grep __abbr9; or true
+abbr | grep __abbr10; or true
+
+# Test renaming to existing abbreviation
+abbr __abbr11 omega11
+abbr __abbr12 omega12
+abbr -r __abbr11 __abbr12; or true

tests/abbr.out

@@ -6,3 +6,5 @@ abbr __abbr1 delta
 abbr __abbr1 delta
 abbr '~__abbr2' '$xyz'
 abbr -- --__abbr3 xyz
+abbr __abbr5 omega
+abbr __abbr8 omega